The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
This disclosure relates to equipment and methods for completing subterranean wells; in particular, wells that produce fluids originating within shale formations.
During completion of a subterranean well, drilling and cementing operations are performed to provide a conduit through which desirable fluids originating within the formation may flow. The cementing operation involves placing a competent cement sheath inside the annular region between the external surface of a tubular body such as well casing, and the borehole wall. The cement sheath supports the casing and provides a hydraulic seal between producing formations. The presence of a hydraulic seal is commonly referred to as zonal isolation.
Operators gain access to desired formation fluids by creating perforations that penetrate the casing and cement sheath, and extend into the formation. Formation fluid then flows into the casing interior and travels through the casing until it reaches a collection facility.
The formation-fluid production rate may be increased by performing stimulation treatments. Such treatments usually involve the injection of fluids through the perforations into the producing formation, with the goal of increasing the formation permeability. The treatments may involve matrix acidizing, hydraulic fracturing or both.
The aforementioned procedures are well known in the industry and have become highly sophisticated after decades of innovation and development. More complete information regarding the aforementioned procedures may be found in the following publications: Economides M C, Watters L T and Dunn-Norman S (eds.): Petroleum Well Construction, John Wiley & Sons (1988); Nelson E B and Guillot D (eds.): Well Cementing, 2nd Edition, Schlumberger (2006); Economides M C and Nolte K G (eds.): Reservoir Stimulation, 3rd Edition, John Wiley & Sons (2000).
Despite the advancements in well-completion technologies, difficulties frequently arise when the industry begins to exploit new types of formations. For example, it has been known for many years that large deposits of natural gas reside in some very low permeability shale formations. Notable examples are the Barnett Shale in Texas, the Woodford Shale which extends from Kansas to West Texas, and the Horn River Basin/Muskwa Shale in British Columbia, Canada. Such formations, whose permeabilities are in the microdarcy range, could not be exploited economically until horizontal-well technologies became widely available in the early 2000s. Today they are commonly called “unconventional gas” reservoirs.
One of the difficulties associated with completing unconventional gas reservoirs is related to hydraulic fracturing operations. The horizontal well usually extends through a shale stratum, and numerous sets of perforations are created along the wellbore to maximize exposure to the gas. Some of the perforations may not be aligned with the minimum stress plane, which defines the direction the fracture will tend to extend outward, away from the wellbore, as the fracturing fluid flows through the perforations. In such cases, the cement sheath often interferes with fracturing-fluid flow. The fluid must find a path around the cement sheath before reaching the preferred flow direction; as a result, there may be a choke effect. This condition is known as “near-wellbore tortuosity,” and is particularly problematic when the producing formation has low permeability. Failure to overcome this problem may significantly limit the effectiveness of the hydraulic fracturing treatment, and reduce the economic viability of the well.
For several years, the industry has overcome the stimulation problem by substituting conventional Portland cements with acid-soluble cements (ASCs). The most common ASCs are magnesium oxychloride (or Sorel) cement or conventional cements embedded with acid-soluble particles such as calcium carbonate. After the well is perforated, a matrix acidizing treatment is performed. The acid dissolves some of the cement around the perforations, thereby minimizing the choke effect and improving the outcome of the subsequent hydraulic-fracturing treatment.
There are some drawbacks associated with the use of ASCs. For example the method requires the step of performing an acidizing treatment before the fracturing treatment; furthermore, there is a risk that the acid may dissolve the cement sheath between two or more sets of perforations, leading to reduced control of the fracturing treatment.
As apparent, despite the valuable contributions of the prior art, it would be beneficial to have techniques that overcome the aforementioned drawbacks.